'Introduction to the immune system' consists of a brief overview of our bodies early response to defend against infections - the topics include the different cells in the immune response, and different types of immunity.
As we know, immunology is the study of how the body defends itself against infection by
micro-organisms, including bacteria, viruses, fungi, and parasites.
It can be broken into the innate and adaptive immune system
Innate – natural immune response, prevents or limits infection
Adaptive – acquired response, eradicates infection
Easy way to remember – innate = natural. Adaptive = acquired.
Contrasts in immune responses:
1. Resistance
Natural innate defence mechanisms are always present and are unaltered when the same
infection returns.
However, if these mechanisms are breached, then the adaptive response is stimulated and
the system is altered, so any future interactions between the host and organism is treated
more quickly and aggressively. This is an acquired response.
To sum up, natural and adaptive immunity work together. The adaptive system memorises
the organism to improve the efficiency of the innate immune system.
2. Effector cells
Innate and adaptive systems employ different effector cells.
Innate – phagocyte cells like neutrophils and macrophages
Adaptive – T cells and B cells
3. Effector molecules
Innate system produces:
- Lysozymes – break down cell walls
- Defensins – cationic peptides that bind to and damage cell walls
- Complement – proteins which enhance bacterial clearance
- Lactoferrin – binds iron and limits growth of iron-dependant bacteria
- Interferons – limit viral replication inside the cell. Type 1 interferons – alpha and
beta and limit viral replication (produced by cell infected by virus. Type 2 interferons
– produced by NK cells and increase macrophage activity in the adaptive response.
Adaptive system produces:
- Antibodies – bind to organisms and get rid of them
- Cytokines – stimulate and recruit other immune cells
, Surface epithelia:
This is basically your skin and it is a natural barrier to infection. It also includes the lining of
the lungs and the GI tract which both come into contact with the environment.
That’s why when there are cuts and burns on the epithelial layer, you are at risk of infection
because more of you is exposed to the environment.
Other physical barriers:
Mucus – prevents bacteria from attaching to lining of the epithelial cells.
Flow of secretions, solid waste or urine all serve to prevent bacterial adherence.
Air flow on the skin prevents bacteria from attaching and reduces bacterial growth
Sweating kills bacteria
Chemical barriers:
Antibacterial peptides such as defensin peptides, and other factors like acidic pH help to
breakdown cell walls, leading to bacterial death.
We have symbiotic bacteria (friendly bacteria) which resides in the GI and urogenital tract.
They compete with harmful bacteria for nutrients and sites of attachment. However,
antibiotics can sometimes kill these symbiotic bacteria, and the harmful bacteria remain,
leading to infection and giving rise to disease, so caution must be taken.
Cells of the innate immune system:
Macrophages – circulate in the blood as monocytes then mature into macrophages.
Important in inflammation, and also stimulating adaptive response. They are phagocytic
Neutrophils (a type of granulocytes) – connected lobe nucleus. Their cytoplasm contains lots
of proteins and enzymes which kill bacteria. Main function is to kill bacteria and
phagocytosis. They are mass produced in bone marrow and are short lived. Most common
WBC. Circulate for about 6 hours. Removed in the spleen if not needed
Eosinophils (another type of granulocyte) – they are low in numbers in circulation, and
increase during infection. They release toxic proteins which kill bacteria. Mainly found in
lungs and urogenital tract.
Mast cells – orchestrate allergic reactions. They respond to allergens (which need to be
removed from the body). Mast cells release granules such as histamine, which dilate and
increase permeability of blood vessels, to stimulate inflammatory response.
NK cells – contain granules containing perforin, which perforates cells, and granzymes,
which are proteases which kill bacteria.
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